1. Field of the Invention
This invention relates to a process for making vinyl pyrrolidone (VP)/vinyl acetate (VA) copolymers, and, more particularly, to improved manufacture of such copolymers having a very low residual VP level, within an advantageously short production cycle.
2. Description of the Prior Art
Potthoff-Karl, in U.S. Pat. No. 5,122,582, described a process for making VP/NA copolymers by copolymerization of the monomers in isopropanol (IPA) solvent in the presence of a radical initiator, steam distilling off the IPA, replacing it with water, (although none of the examples therein include this step) adding more initiator in IPA, post-heating the mixture at about 80° C. for a period of 4½ hours, and again, removing IPA by steam distillation. However, the residual VP content of the copolymer solution obtained was high, e.g. 500 ppm (Ex. 1); 900 ppm (Ex. 2) and 1000 ppm (Ex. 3), based upon the amount of VP employed. Moreover, the duration of the post-treatment step also was excessively long.
Accordingly, it is the object of this invention to provide a new and improved process for making VP/NA copolymers having a very low residual VP level, i.e. <100 ppm, preferably only 50 ppm, in which the post-treatment step is carried out in water only and requires a heating period of only 1–4 hours, preferably only about 2 hours.